Further studies on the isolation, purification and characterization by chemical, biochemical and immunological procedures of biologically active polysaccharides from a number of pneumococcal types and from certain plant materials were carried out. Serological cross-reactions of certain plant polysaccharides was observed in the precipitin tests with rabbit and horse antisera to pneumococcus Types I and III. A polysaccharide isolated from Carthamus tintorius (safflower) also cross reacted with Streptococcus A and D and with Mycoplasma pneumoniae antisera in the complement-fixation test. The properties and reactions of the cross-reactive polysaccharides may be of considerable interest in the possible role of cross-reactive antigens in immunological reactions.